AbstractWe consider brane world compactifications of type I string theory, where O‐planes and D‐branes are wrapped on supersymmetric cycles in Calabi‐Yau 3‐folds. General conditions for supersymmetric vacua are discussed and the concept of intersecting branes that break supersymmetry in a particular weak way is developed. It provides a setting in which one can very concretely engineer gauge groups and chiral fermion spectra according to the features of realistic low energy field theory models of particle interactions. For the example of the well known quintic Calabi‐Yau this is applied to produce a string compactification with Standard Model gauge group and three generation matter spectrum satisfying all string consistency requirements. We also add some comments on the status of anomaly cancellation via the Green‐Schwarz mechanism which is linked to the appearance of Stückelberg masses for abelian gauge bosons. Finally we demonstrate how the branes induce a scalar potential in the effective action via their tension, taking the form of an ${\cal N}=1$ D‐term.